In the field of display devices represented by liquid crystal display devices, since the brightness of the display screen is adjusted in accordance with the intensity of ambient light of the display device (hereinafter, referred to ‘ambient light’), it has been proposed to provide a light sensor in the display device (see patent documents 1 and 2 for example). When the light sensor is provided in a transmission liquid crystal display device, the intensity of the backlight can be enhanced in a bright environment such as exterior, and the intensity of the backlight can be decreased in a relatively dark environment such as nighttime or interior. Thereby, the visibility of the screen is improved, the power consumption is reduced and the lifetime of the liquid crystal display device is increased.
For example, the light sensor can be provided in the liquid crystal display device by mounting a light sensor as a discreet component on a liquid crystal display panel. In a recent experiment, a light sensor is formed monolithically on an active matrix substrate that composes the liquid crystal display panel (see patent document 3 for example). In the latter case, the light sensor is formed on a glass substrate to be a base of the active matrix substrate, at the same time of a process for forming an active element (TFT) or an environmental circuit, by use of the process for forming these TFT and the circuit. According to this process, it is possible to decrease the number of members so as to reduce the manufacture cost, and also provide a smaller display device in comparison with the former case.
For the light sensor of the latter case, for example, a PIN photodiode is known (see Non-patent document 1 for example). This PIN photodiode has a so-called lateral structure. Specifically, the PIN photodiode as disclosed in the above-mentioned Non-patent document 1 has a p-type semiconductor region (p-layer), an intrinsic semiconductor region (i-layer) and an n-type semiconductor region (n-layer) arranged in this order in the surface direction. The p-layer, the i-layer and the n-layer are formed on a polysilicon thin film formed on the glass substrate.
The Non-patent document 1 discloses a research result that the sensitivity of the PIN photodiode can be enhanced by increasing the length of the i-layer (that is, a distance between the p-layer and the n-layer). Therefore, conventionally, the PIN photodiode using the polysilicon thin film is designed to increase the width of the i-layer as much as possible.
Furthermore, the polysilicon thin film composing the PIN photodiode is a thin film common to the polysilicon thin film composing the active element (TFT). It is formed simultaneously in the process of forming the polysilicon thin film composing the active element. According to the recent tendency, for improving the performance of the active element, the active element is formed of a continuous grain silicon (CGS) having electron mobility higher than that of the polysilicon thin film (see patent documents 4-6 for example). In this case, the PIN photodiode is also formed of the CGS thin film.
Patent document 1: JP H04-174819 A
Patent document 2: JP H405-241512 A
Patent document 3: JP 2002-175026 A (FIG. 12)
Patent document 4: JP H06-244103 A
Patent document 5: JP 2001-319878 A
Patent document 6: JP 2003-249447 A
Non-patent document 1: N. Tada and 6 others “A Touch Panel Function Integrated LCD Using LTPS Technology”, Late-News Paper AMD 7-4L, the 11th International Display Workshop 2004, p. 349-350